Power consumption is a critical design parameter in portable electronic devices such as pagers and broadcast frequency modulation (FM) radio receivers. Power consumption is also important in other electronic devices that operate when plugged into an alternating current (AC) supply, of which an example is a radio controlled electric meter reader.
A circuit that is useful in such devices is a frequency prescaler, which is often used within a phase lock loop. In such a use, the phase lock loop generates an RF signal, such as a local oscillator (LO) signal, and the frequency prescaler performs a frequency division of the LO signal to reduce the frequency to a lower frequency that is within an operational range of other circuits used within the phase lock loop. As an example, a frequency division modulus of 5 is sometimes used.
Conventional prescalers such as the one described in U.S. Pat. No. 4,953,187, issued on Aug. 28, 1990 to Herold et al., and entitled "High Speed Prescaler", have a prescaler input characteristic similar to curve 110 shown in FIG. 1, in which the minimum amplitude of the alternating current (AC) component of a prescaler input signal at which the frequency will be prescaled is plotted for a full range 120 of frequencies over which the prescaler could be operated, under a given set of nominal operating conditions, such as a ambient temperature of 25 degrees centigrade and a nominal power supply voltage. The curve 110 has a minimum value 125 which occurs at a free running frequency of the prescaler; that is, the frequency at which the prescaler self oscillates with an input signal having no alternating current (AC) component. When self-oscillating, the output frequency of the prescaler is equal to the free running frequency divided by the modulus of the prescaler. The curve 110 in FIG. 1 is an example input signal characteristic for the prescaler described in U.S. Pat. No. 4,953,187, that has been designed for a given input signal amplitude range and power supply voltage range. The characteristic shifts significantly with respect to frequency in response to variations of power supply voltage (i.e., curve 110 shifts along the frequency axis) and in response to differences in wafer processing (i.e., wafer lot to wafer lot), and also, but to a lesser extent, with temperature. Maximum shifts 113, 115 of the prescaler input characteristic 110 under combinations of voltage, temperature, and process variation are shown in FIG. 1. Because of the significant shift of the input characteristic, an operational frequency range 130 is established for the prescaler that is narrower than the full range 120 possible at one set of operating conditions. In order to ensure that the prescaler will function with a prescaler input signal over the operational frequency range 130, the prescaler input signal is typically amplified in a buffer amplifier stage having a fixed gain chosen to generate an amplitude that is at least the maximum amplitude 140 required over the operational frequency range 130. Although conventional prescalers are designed to use as little power as possible, the requirement for an input signal that has such a large amplitude results in a buffer amplifier that consumes much more power than the prescaler. It will be appreciated that the amplitude 140 is much higher than needed under most operating conditions. While this approach works well, is it generally wasteful of power.
Thus, what is needed is a means for frequency prescaling that substantially reduces the power consumed to operate conventional prescaler circuits.